Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexographic printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. CITR sleeves enable flexographic printing to be more competitive with gravure and offset on print quality.
A typical flexographic printing plate as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
A flexographic printing element is produced from a photocurable printing blank by imaging the photocurable printing blank to produce a relief image on the surface of the printing element. This is generally accomplished by selectively exposing the photocurable material to actinic radiation, which exposure acts to harden or crosslink the photocurable material in the irradiated areas.
The photocurable printing blank typically contains one or more layers of an uncured photocurable material on a suitable backing layer. The photocurable printing blank can be in the form of either a flat, planar element or as a cylindrical printing element.
The printing element is selectively exposed to actinic radiation in one of three related ways. In the first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In the second alternative, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer, which is also sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative. The printing element is then flood exposed through the in situ negative. In the third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these alternative methods is acceptable, with the criteria being the ability to selectively expose the photopolymer to actinic radiation thereby selectively curing portions of the photopolymer.
Thereafter, the imaged and exposed printing element is developed to remove uncured photopolymer on the surface of the printing element and thus reveal the relief image. Development may be accomplished in a variety of ways, including water washing, solvent washing, using an air knife, and thermally, e.g., “blotting.”
Finally, following development, the photopolymer layer may be post-exposed to actinic radiation to provide a more complete cure for the photopolymer layer of the invention and thus a more durable printing plate. The photopolymer layer may also be subjected to a detackification step.
Historically, in flexographic printing, flat, flexible printing plates, fabricated from photopolymers, were hand mounted onto print cylinders by wrapping the printing plate around the cylinder and adhering it there with using various methods such as clamps, tape, magnets or other similar devices. This process works well, but is labor intensive and great care must be taken to ensure that the registration of the plate on the cylinder is accurate. In addition, to allow for additional compression during the print process, a compressible material may be inserted between the print cylinder and the printing plate. A refinement of this process provides for a compressible layer to be contained within the flexographic printing plates
Furthermore, a floor layer, which sets the depth of relief in the photocurable layer, has traditionally been provided in the printing element by back exposing the photocurable layer through the backing layer to create a cured “floor” within the photocurable layer. However, especially in thin plate technologies, back exposure variance can be a problem, resulting in inconsistent imaging results that hold varying degrees of detail with the plate. A further concern is that back exposure is an additional processing step taking additional time to complete. Thus it would be highly desirable to provide a floor layer in a flexographic printing element in a more consistent manner.
U.S. Pat. No. 5,962,111 to Rach, the subject matter of which is herein incorporated by reference in its entirety, describes a compressible printing plate formed by casting liquid photopolymerizable resin directly onto a compressible material. The photopolymerizable resin is then incompletely cured by exposure to actinic radiation. However, Rach only incompletely cures the photopolymerizable resin to create the floor layer and does not address the issue of back exposure variance.
The inventors of the present invention have determined that an improved printing element can be formed with an optional compressible layer and an integral floor layer that provides a more consistent printing element than that of the prior art.
A key advantage of the present invention is that back exposure is no longer required, thus eliminating problems resulting from back exposure variance. Furthermore, there is an obvious time savings resulting from not requiring the back exposure step.
Poor or inconsistent control of light transmittance through the polymer during back exposure can result in the degradation of reverse images, filling them in when compounded with the face exposure sequence. In this instant invention, only face exposure is needed, so the issue of pre-sensitized material or partially cured material during the back exposure sequence causing image degradation is eliminated, resulting in enhanced imaging of the top layer.
Another key benefit of the present invention is the ability to vary the composition of the photocurable layer, provided there is good adhesion between the layers, in order to affect different print attributes, in a manner similar to the way high density and low density foam can affect printing today. A high durometer base, low durometer base, various combinations of the two with the top layer, and even compressible layers (e.g., microspheres embedded with a styrene-isoprene-styrene/styrene-butadiene-styrene copolymer matrix) can be formulated for good compatibility with the surface layer.
The printing plate compositions of the invention can also be formulated to have an extremely fast curing top layer, without concerns for the back exposure sequence being inconsistent. In addition, the front exposure time can be shortened, hold more detail, compounded by the enhanced imaging benefits resulting from the use of a front exposure-only system.
The improved printing elements of the invention also have reduced cold flow in thicker gauges. Higher thickness materials can flow over time, creating havoc in plates sticking to each other during transport (worst case scenario) or small areas of thinner calipers in the plate may stick together after processing, either of which can hurt the print quality.
Finally, printing elements of the invention can be developed such that the top layer itself is one layer or two and may be either analog or digital. The floor can also be formulated to have reduced surface tension properties, aiding in plate clean up.
Finally, exposure of the floor layer through the face and subsequent grinding, sanding, etc. to achieve the specification has been found to be advantageous. Moreover, using this step achieved high adhesion to the base layer surface without the use of primers/adhesives normally used in the art.
In the past, as noted herein, the manufacturers of printing plates formed those printing plates by starting with a sheet of photopolymer laminated to a stable base layer of strong polymer resin, such as polyester. The printing plate manufacturer then partially U.V. exposed the bottom face of the photopolymer through the stable base layer to form a cured floor layer of photopolymer. The top face of the photopolymer layer was then imagewise exposed to U.V. in order to create the relief desired. Thus printing plate manufacturers were forced to use at least two U.V. exposures to create the printing plate, a back exposure to create the floor and a frontal imagewise exposure to create the desired imagewise relief structure. The back exposure is troublesome from at least two points of view. First it is a separate manufacturing step. Second, because of variability in the U.V. transparency of the stable base layer and the need to expose through this layer, non-uniformity in curing the floor layer and variations in floor layer thickness were experienced.
The practice of this invention allows for the use of a new business practice, namely supplying printing plate manufacturers with solid sheet photopolymer with a pre-cured floor layer. This allows the printing plate manufacturer to skip the back exposure step and allows for a more uniform and exacting floor layer to be formed.